The invention relates to a method for coating a substrate in a vacuum chamber having a rotating magnetron, wherein a substrate is guided past the magnetron in a substrate transport direction and is coated by means of a material detached from a target connected to the magnetron, if appropriate also with a reaction of the material with a reactive gas situated in the vacuum chamber.
It is known to produce layers by means of reactive sputtering. In this case, in a high vacuum, a voltage, usually a voltage of alternating polarity, is applied to a target. The target is permeated by a magnetic field, as a result of which the sputtering effect produced by high vacuum and target voltage is made possible or supported. The combination of target and magnetic field generator is designated as a magnetron, and the sputtering is hence designated as magnetron sputtering.
A gas is then introduced into the process space in a targeted manner, said gas reacting chemically with the target material. The reaction product then deposits on a surface of a substrate. This is referred to as reactive sputtering. Oxidic layers, for example, are obtained by introducing oxygen as reaction gas. It is thus possible, for example, to produce a ZnO:Al layer from a Zn:Al target by means of this reactive sputtering.
In practise, so-called tubular magnetrons have proved to be very worthwhile. In this case, a tube-shaped target (tubular target) is provided, in the inner cavity of which the magnetic field generator or the magnetic system is arranged. During the sputtering process, the tubular target is rotated, such that it rotates continuously around the stationary magnetic field. What is thus achieved is that the entire target surface is always processed by the sputtering process. Therefore, unlike in the case of a planar target, it is not possible for zones of different target removal or different target oxidation to form. This ensures, inter alia, that the target is uniformly sputtered, as a result of which better target utilization is achieved.
Tubular magnetrons are usually used in inline vacuum coating installations. These are longitudinally extended vacuum installations comprising a substrate transport system, by means of which substrates are moved through the vacuum coating installation whilst passing through various processing stations, inter alia including tubular magnetron coating stations.
However, such tubular magnetrons can never be manufactured and installed completely cylindrically. In practical use this leads to a fluctuation of the magnetic field at the target surface currently situated above the magnetic system. This is associated with a fluctuation in the operating point.
Operating point is understood here to mean a point on a multidimensional current-voltage characteristic curve set dependent on a plurality of process parameters. A specific desired point or desired region in the characteristic curve set at which the operating point is intended to lie is predefined in order to achieve specific layer qualities, that is to say that the operating point is normally set such that an optimum of layer properties to be obtained is achieved.
Particularly in the case of reactive processes, the influence of the process parameters on the characteristic curve set is particularly strong or not unambiguous, which is manifested in the form of sudden changes or hystereses. This has the effect that minimal fluctuations of the magnetic field which lead to minimal fluctuations in the impedance entail considerable operating point fluctuations.
A fluctuation of the operating point then has the consequence, particularly in the case of reactive processes, that the layer properties on the substrate fluctuate along the transport direction and that the optimum is rarely attained. Parameters of the layer properties can be, for example, transmission and resistance of the deposited layer. In the case of a fluctuation of the operating point, a mixed layer is then deposited at the substrate continuously passing through, with which mixed layer the optimum of transmission and of resistance can never be attained. By way of example, it has not been possible heretofore, during the reactive production of ZnO:Al layers by the tubular magnetron to get beyond an unsatisfactory degree of transmission.
In the process control, the operating point is kept constant if possible. Regulating methods are known for this purpose, for example plasma emission monitoring (PEM), developed by the applicant, or a power regulation by means of reaction gas feeding with a voltage regulated such that it is constant.
During the power regulation, the generator used for providing the target voltage is operated in a voltage-regulated fashion and the desired power is set by means of the reactive gas flow rate, in particular the oxygen flow rate. When a tubular magnetron is used, during the tube rotation the changes in the magnetic flux at the target surface can result in the readjustment of the reactive gas flow rate, in particular of the oxygen flow rate, while the power can be kept very constant.
However, the regulation of the reactive gas flow rate is also associated with the change of other variables, when producing a ZnO:Al layer the intensity of the Zn line in the optical emission spectrum. That has the effect that, despite very good constancy of the voltage and the power, the operating point is varied by way of the tube rotation and the mixed layers already described occur, in which good properties are combined with poor properties.
If, by way of example, a specific resistance is intended to be set, then said resistance is determined by the high-resistance portion in the layer. Consequently, a limit value of the resistivity has to be undershot in the entire layer. However, the rotation has the effect that one part of the layer is deposited with high resistance but, upon further rotation of the tube, another part is also deposited with low resistance.
Therefore, the operating point has to be tracked by means of the tube rotation, which is not successful solely with oxygen flow rate and power regulation.
The object of the invention, then, consists in improving the homogeneity of the layer on a substrate by means of the target rotation.